Soybean consumption has been associated with a reduced incidence of breast cancer, although its mechanism of action is unknown. We showed that consumption by females for one month of soymilk containing significant quantities of the isoflavones, daidzein and genistein, effectively reduced levels of endogenous ovarian steroids and adrenal androgen, which are recognized risk factors for breast cancer. Reduced levels of these steroid hormones may reduce breast cell proliferation and these results may explain the protective effects of soya diets against breast cancer development. However, long term ingestion by sheep of high-isoflavone forages reduced the fertility of sheep. Our long term objective is to determine the effective and safe frequency of consumption needed for positive health benefit:risk ratio. A randomized unblinded study (20 subjects/arm) with three arms will determine if a 5-month consumption by regularly cycling women of soya diets containing 0 or >200 mg of isoflavones will prolong menstrual cycles in addition to reducing levels of ovarian hormones and gonadotropins compared to consumption of none soya diets. The levels of their ovarian hormones, gonadotropins, and urinary metabolites of phytoestrogens, the menstrual cycle lengths, daily basal body temperatures, vaginal cytology, and endometrial thickness, and their interrelationship, will be determined before, during and after termination of soya feeding. This and other ongoing studies will optimize the frequency of phytoestrogen consumption which will help design future trials to determine if a soya diet can favorably influence breast cancer risk in American women.